1. Field of the Invention
This invention relates to the field of silver halide emulsion manufacture and particularly to silver halides made by the so-called "splash-precipitation" method. Still more particularly, this invention relates to a method for making splash-prepared silver halide emulsions with a uniform particle size distribution of the silver halide crystals.
2. State of the Art
Preparation of sensitive silver halides for a photographic emulsion is a complex process. Basically the silver halide crystals can be prepared by three well-known methods: the single jet method; the "splash" method; or the double jet or balanced double jet (BDJ) method. In the single jet method, all of the halide solution is placed in the mixing vessel together with the protective colloid (e.g., gelatin) right from the start and the silver nitrate solution is then added to this mixture over a relatively long period of time, e.g., 20 to 25 minutes. This procedure is exemplified by Corben et al. U.S. Pat. No. 4,339,532, Example 14. In the so-called "splash" method for making silver halide crystals, the silver nitrate solution is added in a series (e.g., 2-4) of finite "splashes" to the vessel containing the halide and protective colloid. Conventionally, some ammonia is present in one or more of these silver solutions. In the BDJ method, the halide solution and the silver nitrate solution are added simultaneously to a solution of gelatin in the mixing vessel. Conventionally a small amount of halide solution may be present in the mixing vessel prior to the start of BDJ addition.
The splash precipitation process usually creates an increased number of so-called "crystal lattice defects" in which a number of ions are not positioned correctly in the silver ion-halide ion network. Emulsions made from these crystals can be used to make films which exhibit improved sensitometry, especially improved speed, as compared with films made from silver halide emulsions with crystals that have few or no lattice defects or disorientation. In spite of this advantage, it is difficult to make splash-prepared emulsions with a uniform distribution of particle sizes. In many areas of photography it is important that emulsions have this uniform distribution in order to control gradient. For example, in the field of medical x-ray, the emulsions used to prepare these films are conventionally prepared by splash or single jet procedures. These procedures produce a large number of crystal defects but because of the nonuniformity of the particle size distribution also have lower gradient. On the other hand, production of silver halide crystals by the BDJ process will yield crystals having good uniformity of particle size but these crystals generally lack the disorientation or lattice defects and thus will not be as inherently fast as splash-prepared emulsions.
It is also known to use a seed emulsion with a known particle size, said seed emulsion or crystal being added during the preparation of BDJ or single jet emulsions. In a BDJ procedure silver halide forms on these seeds and produces a uniform final crystal. This process has not been used for the splash preparation of crystals since it was thought that crystals with a narrow particle size distribution and a high internal defect density could not be produced thereby.